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Oscillator Selection for ST62 AN670 Application note Oscillator selection for ST62 Introduction The purpose of this note is to give indications on how to choose a resonator or a quartz crystal in order to achieve reliable oscillation with the ST62 Microcontroller. This document provides first the major resonator parameters useful for a design. It then proposes measurement methods to ensure a safe oscillation. October 2008 Rev 2 1/12 www.st.com Contents AN670 Contents 1 Oscillation frequency . 3 2 Oscillation conditions . 4 2.1 Barkhausen criteria . 4 2.2 Start-up . 4 2.3 Measurement of the loop gain (open loop) . 5 2.4 Frequency stability . 5 2.5 Start-up time . 6 3 Conclusion . 7 Appendix A Test of a CSA Murata crystal resonator with an ST6210xx . 8 A.1 Choice of the network capacitances . 8 A.2 Pseudo closed loop measurement . 8 A.3 Start-up time . 8 A.4 Conclusion. 8 Appendix B Calculation of the resonant frequency of ceramic resonator . 9 B.1 Equivalent circuit at the resonance frequency. 9 B.2 Transformation for simple calculation . 9 B.3 Resonant frequency . 9 B.4 Note. 10 4 Revision history . 11 2/12 AN670 Oscillation frequency 1 Oscillation frequency The resonator can be modelised by a serial/parallel oscillator circuit as described in Figure 1. The additional capacitances Cext are usually connected to the oscillator pins in order to define a stable oscillating frequency. The value of these capacitances is usually given by the manufacturer of the resonator. The oscillation frequency is the resonant frequency of the equivalent circuit given in Figure 2. The resonator is inductive in the oscillation frequency range. Figure 1. Resonator model Figure 2. Equivalent circuit Co R L R Co L C1 C1 Cext 3/12 Oscillation conditions AN670 2 Oscillation conditions The proposed method is based on the Barkhausen criteria. This leads to a safe result providing that the oscillator fulfills these criteria. Three points have to be analysed: oscillator start-up, frequency stability and the start-up time. 2.1 Barkhausen criteria An oscillator can be modelized as defined in Figure 3. B is the resonator gain and G the amplifier/inverter gain. The value of BxG defines the oscillator behaviour: ● BxG >> 1: square waveform, start-up OK ● BxG > 1: waveform with harmonic distortion, start-up OK ● BxG = 1: sine waveform, start-up critical ● BxG < 1: no oscillation Figure 3. Oscillator model G G: Inverter/Amplifier Input Output B: Resonator circuit B 2.2 Start-up The oscillator can start if the gain BxG is above 1. The amplifier gain must compensate for the resonator circuit attenuation and provide a sufficent gain margin (>3 dB). In addition, the resonator circuit B must introduce a 180 ° phase delay if the G amplifier is an inverter and no rotation if it is a non inverting amplifier. With classical circuits such as a Pierce type oscillator (Figure 4.), the 180 ° phase rotation is due to capacitances (Cout and Cin). Figure 4. Pierce type oscillator Figure 5. Equivalent schematic at the resonant frequency Vin Vout LRCo Cin Cout + - Cin - + Cout 4/12 AN670 Oscillation conditions At the resonance frequency (serial mode), the circuit can be modelized as described in Figure 5. The resonator voltage is balanced between the two capacitances. So the phase of the voltages Vin and Vout is delayed by 180 °. Depending on the capacitance values, Vin is either higher (Cout>Cin), smaller (Cout<Cin) or equal to Vout. The trade offs in the choice of Cin and Cout are: ● Cout = Cin : Vin = Vout This is the typical case and is to be used as often as possible. ● Cout > Cin : Vin > Vout The loop gain is increased but there is a risk that the oscillation occurs at a harmonic of the resonator frequency. ● Cout < Cin : Vin < Vout The output voltage is increased. The risk of oscillation at a harmonic of the resonator frequency is low. Vin must be high enough to satisfy the condition BxG >1. 2.3 Measurement of the loop gain (open loop) The measurement is based on the schematic shown in Figure 6. The method is the following: 1. Open the loop as described in Figure 6. 2. Place an oscillator probe on points 1 and 2. Note that the real C value for the calculation is Cprobe + Cin. 3. Inject a voltage S with a signal generator. This signal must be adjusted in frequency to maximize the voltage V2. 4. Adjust S to a value small enough to avoid saturation of the amplifier (around 200 mV on V1). 5. Calculate the ratio V1t/V2. This value has to be between +3 and +10 dB (1.5 to 3). If the ratio is above +3 dB, the oscillator start is safe. If it is below, Cin should be decreased. Figure 6. Gain loop measurement schematic V1 OSCout OSCin S V2 Cprobe Cin Cout 2.4 Frequency stability The stability is first defined by the resonator characteristics. Nevertheless, if the open loop gain exceeds +10 dB, the oscillation could occur on a harmonic of the resonator frequency. In such cases, the value of Cin should be increased to reduce the loop gain or a filter rejecting this harmonic must be added. 5/12 Oscillation conditions AN670 2.5 Start-up time The start-up time depends on the amplifier polarisation time and on the circuit transients. The polarisation of the amplifier can be accelerated by dividing the Cin capacitance in two as described in Figure 7. Figure 7. Amplifier polarisation acceleration Vcc Cin 2 Cin Cout 2 The start-up time is also longer when Cin and Cout are increased. As a result, for very low start-up time (i.e. low frequency quartz crystal), these capacitances values should be as small as possible. Generally, the higher the crystal Q factor and lower the crystal frequency, the longer the start-up time. 6/12 AN670 Conclusion 3 Conclusion This note describes a method to choose oscillator network capacitances adapted to standard resonators and quartz crystals (i.e. rs < 60 ohms and gain > 500). Since several network values can be chosen, the capacitances values should be minimized in order not to affect the resonance frequency and reduce the start-up time. 7/12 Test of a CSA Murata crystal resonator with an ST6210xx AN670 Appendix A Test of a CSA Murata crystal resonator with an ST6210xx A.1 Choice of the network capacitances Resonator equivalent values: L = 385 µH C0 = 4.4 pF C1 = 36.3 pF rs = 8.7 ohm Q = 1134 The oscillation mode is the fundamental mode. The recommended load capacitances for 4 MHz oscillation frequency are 2x30 pF. The corresponding oscillation frequency as calculated from the formula given in Appendix B is 4.03 MHz. A.2 Pseudo closed loop measurement In the worst case (Tambiant max, Vsupply min) the gain Vout/Vin is 4.8. So the safety margin is +13.6 dB. A.3 Start-up time The start-up time is measured in closed loop. In the worst conditions (Tambiant max, Vsupply min), it is less than 1 ms. A.4 Conclusion The selected ceramic resonator matches with the ST6210 oscillator. 8/12 AN670 Calculation of the resonant frequency of ceramic resonator Appendix B Calculation of the resonant frequency of ceramic resonator B.1 Equivalent circuit at the resonance frequency Figure 8. Equivalent circuit R Co L C1 C2 C3 B.2 Transformation for simple calculation Figure 9. Transformed circuit RLCo C1 C2 C3 RL C2 x C3 Where Ceq = C1 + C2 + C3 Co Ceq L Co x Ceq Where C' = Co + Ceq C' B.3 Resonant frequency 1 f = -------------------------- 2π LxC′ 9/12 Calculation of the resonant frequency of ceramic resonator AN670 B.4 Note When using a ceramic resonator, the oscillation frequency is usually between the parallel and the series resonances. So both C1 and Co have ot be included in the calculation. The resonance frequency of a crystal resonator is very near to the serial frequency. So only Co has to be used for the frequency calculation. 10/12 AN670 Revision history 4 Revision history Table 1. Document revision history Date Revision Changes February-1994 1 Initial release. Format changed. 03-Oct-2008 2 Title of Appendix A modified (ST6210xx instead of ST6210). Logo and disclaimer updated. 11/12 AN670 Please Read Carefully: Information in this document is provided solely in connection with ST products. STMicroelectronics NV and its subsidiaries (“ST”) reserve the right to make changes, corrections, modifications or improvements, to this document, and the products and services described herein at any time, without notice. All ST products are sold pursuant to ST’s terms and conditions of sale. Purchasers are solely responsible for the choice, selection and use of the ST products and services described herein, and ST assumes no liability whatsoever relating to the choice, selection or use of the ST products and services described herein. No license, express or implied, by estoppel or otherwise, to any intellectual property rights is granted under this document. If any part of this document refers to any third party products or services it shall not be deemed a license grant by ST for the use of such third party products or services, or any intellectual property contained therein or considered as a warranty covering the use in any manner whatsoever of such third party products or services or any intellectual property contained therein. UNLESS OTHERWISE SET FORTH IN ST’S TERMS AND CONDITIONS OF SALE ST DISCLAIMS ANY EXPRESS OR IMPLIED WARRANTY WITH RESPECT TO THE USE AND/OR SALE OF ST PRODUCTS INCLUDING WITHOUT LIMITATION IMPLIED WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE (AND THEIR EQUIVALENTS UNDER THE LAWS OF ANY JURISDICTION), OR INFRINGEMENT OF ANY PATENT, COPYRIGHT OR OTHER INTELLECTUAL PROPERTY RIGHT.
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